This application relates to a refrigerant system that utilizes an expander to provide a more efficient expansion process and to recover at least a portion of energy from this expansion process, and wherein at least partially expanded refrigerant is injected back into the compression process to reduce discharge temperature.
Refrigerant systems utilize a refrigerant that is usually circulated through a closed-loop refrigerant cycle to condition a secondary fluid, such as air, water or glycol solution, to be delivered to a climate controlled environment. Typically, in a basic refrigerant system, a compressor compresses the refrigerant and delivers it to a first heat exchanger, which is a condenser, for subcritical applications, and a gas cooler, for transcritical applications. In the first heat exchanger, heat is rejected from the refrigerant and is absorbed by another secondary fluid, such as ambient air. Refrigerant, from that first heat exchanger, passes through an expansion process, during which its pressure and temperature are reduced. Downstream of the expansion device, a refrigerant passes through a second heat exchanger, or so-called evaporator, and then back to the compressor. In the second heat exchanger, the refrigerant is evaporated and typically superheated, while cooling a secondary fluid to be delivered to a climate controlled environment.
Expansion devices can be of a so-called passive or active type. Passive expansion devices are typically represented by an orifice or a valve. The refrigerant expansion through these devices follows an inefficient isenthalpic thermodynamic process. One option that is known in refrigerant systems is the use of an active expansion device or an expander, which follows more thermodynamically efficient isentropic process and can be a turbine, a piston expander (a free-piston type or a linked-piston type), a scroll expander, a screw expander or any other type expander expanding the refrigerant from a higher pressure to a lower pressure. The expander typically recovers at least portion of energy from the expansion process, and that portion of energy is utilized, at least partially, to drive other components, such as, for example, one of the refrigerant system components, such as a compressor, a fan or a pump. Expanders increase performance (efficiency and capacity) of a refrigerant system by recovering this portion of energy and by utilizing a more thermodynamically efficient isentropic expansion process.
Another feature utilized in refrigerant system designs is the injection of a portion of refrigerant back into the compression process. The refrigerant injected into the compression process is typically a two-phase mixture of vapor and liquid. The liquid-vapor mixture cools the compressor elements as well as the main flow of compressed refrigerant. The cooling of the main refrigerant flow occurs as this cooler two-phase mixture undergoes evaporation and mixing process with the hotter main refrigerant vapor.
The refrigerant injection cooling feature becomes even more important for high-pressure refrigerant systems, and specifically for refrigerant systems operating in a transcritical cycle, such as CO2 refrigerant systems. These refrigerants operate at higher pressures and often higher pressure ratios than formerly used conventional refrigerants, thus promoting higher discharge temperatures. Additionally, in most cases, the transcritical cycle is less efficient than the conventional subcritical cycle, requiring even higher discharge pressures or other means of performance enhancement, such as, for instance, a liquid-suction heat exchanger, both increasing refrigerant discharge temperatures. Specifically, CO2 refrigerant has a higher value of a polytropic exponent (than other conventional refrigerants), also undesirably affecting the discharge temperature. On the other hand, in the transcritical cycle, pressure and temperature are essentially independent from each other, so discharge temperature reduction may find additional benefits of performance optimization, operational envelope extension and reliability improvement.
The use of this injected refrigerant is known in the art, and a worker skilled in this art would recognize when and how much injected refrigerant would be desirable. For instance, the refrigerant may be injected into the compression process to maintain a compressor discharge temperature, to control discharge or suction superheat, etc.
However, a refrigerant system has never been proposed, which has utilized the expander in combination with this refrigerant injection cooling, especially in conjunction with CO2 applications that can operate in a transcritical cycle.